Reduce time in processing multiple excel sheets

Need a macro, which combines multiple excel sheets (csv format) from different folder and adds the name of the folder in a column of the excel sheet.

Basically, in path D:\ , i have the following folders
Maths, physics,Biology, English, French

Now i would place excel sheets belonging to each folder. The number of excel sheets can vary.
for the sake of convenience suppose we have 3 excel sheets in each folder.

Now the macro is to combine all the excel sheets from all the folders into a single excel sheet.
Problem comes that there is no way to distinguish between the excel sheets once they are combined.
So which picking up the data from each excel sheet, need to add the text " Math" or "physics" ( depending on the folder from which the excels are extraced) in a column at the end of the sheet.

Now I have a working macro that does all of the above.
however, i need to refine the logic or processing speed for this. As the total number of rows, after combining all the sheets can exceed 250000 rows. The current macro takes 20 minutes to process the data and provide output.

Can you please advise how i can reduce the time or a different logic that can reduce the time when dealing with these many rows of data?

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Getting a compile error: user defined type not defined..

In the VBA IDE, go to Tools > References
Tick "Microsoft Scripting Runtime" and click OK
and Run it.

The code has limits, as mentioned, Im dealing with rows that exceed 250000, would like a macro that can process any number of rows and not specify a limit such as 65554 or so.

I tried using LastRow = Range("E" & Rows.Count).End(xlUp).Row, but it is giving errors.. please help..